1. Field of the Invention
The invention relates to a transceiver method in a radio system in which several antennas are used in the transmission and/or reception of a signal propagating through a radio channel, and a radio system implementing the method.
2. Description of the Related Art
In telecommunication connections, the transmission path used for transmission of signals causes interference for telecommunication, irrespective of the physical form of the transmission path. The transmission path can be a radio link, an optical fibre or a copper cable, for example. In telecommunication utilizing radio links, in particular, situations often arise where the quality of the transmission path varies from one connection to another, and also during the connection. One typical phenomenon causing changes in the transmission channel is fading on the radio path. Other simultaneous connections may also cause interference that may vary in the function of time and place.
Using diversity in the transmitter has been used as one solution to this problem. Diversity methods generally used include time, frequency and antenna diversity, for instance. In time diversity, interleaving and coding are used, with which time diversity is provided for the signal to be transmitted, but the disadvantage is that there will be delays in the transmission, particularly when the channel is slowly fading. In frequency diversity, the signal is transmitted at several frequencies simultaneously. However, the method is inefficient when the coherence bandwidth of the channel is great. In antenna diversity, there is more than one antenna in the signal transmission and/or reception. Thus, the signal components multi-propagated through different channels are not, in all probability, interfered by simultaneous fading.
Multiple antenna techniques, such as antenna or transmission diversity, the use of array antennas or beam forming, allow improvement of performance and improvement of capacity and coverage of the uplink and downlink, for example when time division multiple access (TDMA) and wideband code division multiple access (WCDMA) are used.
Antenna diversity can be divided into receiving and transmitting diversity. In receiving diversity, two or more antennas whose positions or polarizations differ from each other are used for receiving the transmitted signal. In transmission diversity, the same signal is transmitted to the receiver by using two or more different antennas. In prior art solutions, antenna diversity is more generally used in the downlink of a radio system, because in such a case the user equipment does not have to be provided with several antennas. Requirements for the complexity of the user equipment have, in general, been strict, so that prior art solutions tend to use multiple antenna systems based on algorithms executed in a base transceiver station instead of algorithms performed in user equipment.
A known example of such a method is beam forming, with which average improvement of the performance of a radio link is achieved. The aim of beam forming is to increase the average strength of the electric field close to the user equipment by transmitting the same signal with antennas correlating intensively in such a way that the signals are summed up in the direction of the user equipment. The operation of this conventional beam forming is based on determining the average direction of the user equipment.
The MIMO (multiple input, multiple output) method is presently one of the multiple antenna methods having gained wide interest. In this method, a signal is transmitted to a receiver by using two or more different antennas, and the transmitted signal is received by using two or more different antennas. The MIMO is described in more detail in, for instance, the publication by G. J. Foschini, “Layered Space-Time Architecture for Wireless Communication in a Fading Environment When Using Multi-Element Antennas”, Bell Labs Technical Journal, Autumn 1996, which is incorporated by reference herein. Good performance can be achieved with the MIMO if the signals transmitted and received through different antennas travel through different channels. The problem with MIMO methods has been that the channels must be rather uncorrelated relative to each other in order for the MIMO to function well. In many cases, correlation results from a situation where there is a line-of-sight (LOS) between the base transceiver station and the user equipment, in other words the line from the transmitting antenna to the receiving antenna is unrestricted. The drawback of the MIMO method is thus that it does not function well in line-of-sight situations. A prerequisite for good performance is also that the user equipment of the radio system comprises at least two antennas.
In telecommunication connections, the intention is to transmit a signal in as an errorless manner as possible and, at the same time, to transmit information as efficiently as possible; in other words, the aim is to utilize the capacity of the transmission channel as efficiently as possible in data transmission. The object of interest is particularly the transmission rates achieved in designing cellular radio systems. The third generation and the more recent generations of mobile systems allow arrangement of the transmission rate according to the information to be transmitted. For example, speech can be transmitted at a lower rate than data, and, the highest transmission rate possible can be used for multimedia applications. In MIMO methods, the transmission rate can be increased by sending several different signals simultaneously at the same frequency. Conventionally, the use of diversity and the use of several signals to increase the transmission rate have been alternatives that have excluded each other.
High transmission rates also usually require high transmission power in order for the desired connection quality to be achieved. In addition to the compensation of the effects of the propagation environment, also the so-called spectral efficiency, i.e. achieving the required quality by using radio resources as little as possible, typically at as low a transmission power as possible, affects performance of the transmission path.
The problem with previous methods have been that in some cases, due to the incompatibility of the polarizations of the transmitter and the receiver, the signal does not reach its destination even if the transmission power is increased. A corresponding problem may arise when signals are sent from antennas between which there is correlation.